linux-kernel - Re: [PATCH v9 29/30] KVM: arm64: Add selftest event support to nVHE/pKVM hyp

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Message-ID: <86ms24bims.wl-maz@kernel.org>
Date: Fri, 23 Jan 2026 12:47:39 +0000
From: Marc Zyngier <maz@...nel.org>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: rostedt@...dmis.org,
	mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com,
	linux-trace-kernel@...r.kernel.org,
	oliver.upton@...ux.dev,
	joey.gouly@....com,
	suzuki.poulose@....com,
	yuzenghui@...wei.com,
	kvmarm@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	jstultz@...gle.com,
	qperret@...gle.com,
	will@...nel.org,
	aneesh.kumar@...nel.org,
	kernel-team@...roid.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 29/30] KVM: arm64: Add selftest event support to nVHE/pKVM hyp

On Fri, 23 Jan 2026 12:21:04 +0000,
Vincent Donnefort <vdonnefort@...gle.com> wrote:
> 
> On Fri, Jan 23, 2026 at 12:14:34PM +0000, Vincent Donnefort wrote:
> > On Wed, Jan 07, 2026 at 03:40:22PM +0000, Marc Zyngier wrote:
> > > On Tue, 02 Dec 2025 09:36:22 +0000,
> > > Vincent Donnefort <vdonnefort@...gle.com> wrote:
> > > > 
> > > > Add a selftest event that can be triggered from a `write_event` tracefs
> > > > file. This intends to be used by trace remote selftests.
> > > > 
> > > > Signed-off-by: Vincent Donnefort <vdonnefort@...gle.com>
> > > > 
> > > > diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> > > > index 37c015b54ff6..e2de7ad64938 100644
> > > > --- a/arch/arm64/include/asm/kvm_asm.h
> > > > +++ b/arch/arm64/include/asm/kvm_asm.h
> > > > @@ -96,6 +96,7 @@ enum __kvm_host_smccc_func {
> > > >  	__KVM_HOST_SMCCC_FUNC___tracing_update_clock,
> > > >  	__KVM_HOST_SMCCC_FUNC___tracing_reset,
> > > >  	__KVM_HOST_SMCCC_FUNC___tracing_enable_event,
> > > > +	__KVM_HOST_SMCCC_FUNC___tracing_write_event,
> > > >  };
> > > >  
> > > >  #define DECLARE_KVM_VHE_SYM(sym)	extern char sym[]
> > > > diff --git a/arch/arm64/include/asm/kvm_hypevents.h b/arch/arm64/include/asm/kvm_hypevents.h
> > > > index 268b3cd7a1b2..c276fd28e0bf 100644
> > > > --- a/arch/arm64/include/asm/kvm_hypevents.h
> > > > +++ b/arch/arm64/include/asm/kvm_hypevents.h
> > > > @@ -42,4 +42,15 @@ HYP_EVENT(hyp_exit,
> > > >  	),
> > > >  	HE_PRINTK("reason=%s", __hyp_enter_exit_reason_str(__entry->reason))
> > > >  );
> > > > +
> > > > +HYP_EVENT(selftest,
> > > > +	HE_PROTO(u64 id),
> > > > +	HE_STRUCT(
> > > > +		he_field(u64, id)
> > > > +	),
> > > > +	HE_ASSIGN(
> > > > +		__entry->id = id;
> > > > +	),
> > > > +	RE_PRINTK("id=%llu", __entry->id)
> > > 
> > > Not strictly related to this patch, but I find that the trace itself
> > > lacks context. For example:
> > > 
> > > [001]	  323.847422: hyp_enter reason=hvc
> > > [001]	  323.847423: hyp_exit reason=eret_host
> > > [001]	  323.847688: hyp_enter reason=hvc
> > > [001]	  323.847688: hyp_exit reason=eret_host
> > > [001]	  323.847706: hyp_enter reason=hvc
> > > [001]	  323.847707: hyp_exit reason=eret_host
> > > [001]	  323.847722: hyp_enter reason=hvc
> > > [001]	  323.847723: hyp_exit reason=eret_host
> > > 
> > > That's all fine as long as I'm dealing with a single guest, or even
> > > with a single vcpu. Trying to trace multiple guests, or even multiple
> > > vcpus makes the whole thing completely unusable, because I have no
> > > idea what I'm looking at.
> > > 
> > > To make this useful, having some context provided by the host really
> > > is required.
> > 
> > I could add to the event header the VM pid related to the currently loaded vCPU
> > (if any). I can access it easily with host_kvm->userspace_pid. WDYS?
> 
> Or actually directly kvm_vcpu->pid?

More like it indeed.

	M.

-- 
Without deviation from the norm, progress is not possible.